In the HARQ (short for Hybrid Automatic Repeat Request) mode, the code word sent by a sending terminal can not only detect error but also have certain error correcting capability. After receiving the code word, the decoder of a receiving terminal first detects error situation, and then automatically corrects error if the situation is within the error correcting capability of the code word, or the receiving terminal sends a decision signal to the sending terminal via a feedback channel and requires the sending terminal to re-transmit information if there are too many errors and the situation is beyond the error correcting capability of the code word while the code word can detect the error. In OFDM (short for Orthogonal Frequency Division Multiplex) systems, ACK/NACK (short for Acknowledgement/Negative-acknowledgement) messages are used to indicate ACK/NACK transmissions, thereby deciding whether repeating is necessary.
In LTE (short for Long Term Evolution) systems, the ACK/NACK response messages can be separately sent on a PUCCH (short for Physical Uplink Control Channel), and can also be sent on a PUSCH (short for Physical Uplink Shared Channel) together with data.
In FDD (short for Frequency Division Duplex) systems, as uplink subframes correspond to downlink subframes one by one, when a PDSCH (short for Physical Downlink Shared Channel) only contains one code word stream, a UE (short for User Equipment) needs to feed back a 1-bit ACK/NACK response message, when the PDSCH contains two code word streams, the UE needs to feed back a 2-bit ACK/NACK response message, when the UE does not have PUSCH for sending in a current subframe, the UE will use a format 1a/1b on the PUCCH to send the 1/2 bit ACK/NACK response message; when the UE has the PUSCH for sending in the current subframe, the UE will subject the information of 1/2 bit to mapping from a certain ACK/NACK state to corresponding bit, channel coding, scrambling and modulating, then multiplex it with data and send them on the PUSCH.
In TDD (short for Time Division Duplex) systems, as uplink subframes do not correspond to downlink subframes one by one, that is, the ACK/NACK response messages corresponding to multiple downlink subframes need to be sent on the PUCCH/PUSCH of one uplink subframe, wherein the downlink subframes corresponding to the uplink subframes are integrated to form a bundling window. In the LTE TDD, two ACK/NACK transmissions modes are defined, which will be described hereinafter respectively.
One is an ACK/NACK bundling method, and the basic idea of this method is to subject the ACK/NACK messages of the code word streams that correspond to respective downlink subframes and need to be fed back in the uplink subframe to logical AND operation, if the PDSCH transmission of one downlink subframe contains two code word streams, the UE will feed back a 2-bit ACK/NACK response message that has been subjected to subframe logical AND operation and corresponds to each of the code word streams, if the PDSCH transmission of the respective subframes only contains one code word stream, the UE will feed back a 1-bit ACK/NACK response message that has been subjected to subframe logical AND operation and corresponds to the code word stream, when the UE does not have PUSCH for sending in a current subframe, the UE will use a format 1a/1b on the PUCCH to send the 1/2 bit ACK/NACK response message; when the UE has the PUSCH for sending at the current subframe, the UE will subject the information of 1/2 bit to channel coding and channel interleaving, then multiplex it with data and send them on the PUSCH.
The other is an ACK/NACK multiplexing method, in this method, an ACK/NACK response message will be fed back to each of the downlink subframes, and multiple ACK/NACK response messages should be fed back corresponding to multiple subframes, and when the PDSCH transmission of a certain downlink subframe contains 2 code word streams, the ACK/NACK response message of each of the code word streams will be first subjected to logical AND operation, and finally each downlink subframe only correspond to one ACK/NACK response message. When the UE does not have PUSCH for sending on a current subframe, the UE will use a format 1b with channel selection on the PUCCH to send the multiple ACK/NACK response messages. This method can carrier at most 4 bits of ACK/NACK response messages by combining channel selection with PUCCH format 1b. In addition, for different numbers of ACK/NACK, the LTE TDD defines a mapping table of different ACK/NACK states to modulation symbols sent at a selected PUCCH channel in format 1b. When the current UE has PUSCH for sending in the current subframe, the UE will subject the multiple bits of information to mapping from a certain ACK/NACK state to corresponding bit, channel coding and channel interleaving, then multiplex it with data and send them on the PUSCH. The number of the ACK/NACK that the UE needs to feed back at each subframe is decided by the uplink/downlink subframes configuration of the system.
The UE is configured by the higher layer signaling whether the UE uses the ACK/NACK bundling or the ACK/NACK multiplexing to feed back the ACK/NACK.
To meet requirements of the ITU-Advanced (short for International Telecommunication Union-Advanced), as an evolution standard of the LTE, the LTE-A (short for Long Term Evolution Advanced) system needs to support a wider system bandwidth (up to 100 MHz) and should also be backward compatible with the current standards of the LTE. Based on the current LTE systems, the bandwidths of the LTE systems can be combined to obtain a wider bandwidth, and this technology is called a CA (short for Carrier Aggregation) technology, which can improve the spectrum efficiency of an IMT-Advance system, relieve the lack of spectrum resources and thus optimize the use of spectrum resources.
When the LTE-A uses the CA technology, and when a base station configures multiple downlink component carriers, the UE needs to feed back the ACK/NACK response messages of the code word streams corresponding to the multiple downlink component carriers. In the LTE-A, when the ACK/NACK response message is sent on a PUCCH, two feedback manners are defined: using PUCCH format 1b with channel selection, and a feedback manner based on DFT-s-OFDM.
As described above, for the method using the PUCCH format 1b with channel selection, for different ACK/NACK feedback numbers, the corresponding mapping tables are different, and in one mapping table, different order of the ACK/NACK response messages are different will result in different selected channel and modulation symbol for final feedback. Currently, in the discussion of the LTE-A, the corresponding relationship between the ACK/NACK response messages of the code word streams corresponding to the downlink component carriers and the HARQ-ACK states in the mapping tables is still under discussion.
For the method based on DFT-s-OFDM, for different ACK/NACK feedback numbers, the mappings from corresponding ACK/NACK states to corresponding bits are also different. Moreover, for the number of the ACK/NACK that the UE needs to feed back, the UE has an agreed relationship with the base station. Currently a supposed work of the LTE-A is that the ACK/NACK fed back by the UE is determined according to the configured downlink component carriers and the transmission mode of each of the downlink component carriers. For example, if a base station configures 3 carriers for a terminal and the 3 carriers are all configured at an MIMO mode, the UE needs to feed back an ACK/NACK response message of (3×2=6) bits. However, currently there is no any detailed solution for the mapping relationship between the 6-bit information as fed back and each configured component carrier.
Therefore, if the mapping relationship between the ACK/NACK response messages and the bits as fed back or the corresponding relationship between the ACK/NACK response messages and the HARQ-ACK states in the mapping tables are understood in different ways at the base station and the terminal, the base station understands the ACK/NACK response messages fed back by the UE in a wrong way, and then error is incurred.